1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus, such as a full-color printer, or the like, more specifically relates to an image forming apparatus having a toner image forming unit that forms a not-yet-fixed image on a recording material, and a fixing apparatus that performs electromagnetic induction-heating due to an operation of magnetic fluxes generated by a magnetic flux generating unit, in order to fix the not-yet-fixed toner image on the recording material.
2. Description of Related Art
In the electrophotographic type image forming apparatus, an exposure unit exposes a photosensitive drum charged by a charging unit to thereby form an electrostatic latent image, and a developing unit develops the electromagnetic latent image to thereby form a toner image, a transfer unit transfers the toner image on to a recording material, and a fixing apparatus heats and presses transferred recording material to thereby fix the toner image on to the transfer material. This fixing unit has a fixing roller contacting the toner image on the recording material, and a pressure roller forming a nip while contacting the fixing roller, which nips and conveys the recording material.
This fixing roller has been generally of a halogen heater-used heating type; however, in these days, an electromagnetic induction heating type fixing roller has been manufactured for generating an eddy current due to magnetic fluxes to thereby generate heat, as described in Japanese Patent Application Laid-open No. 62-150371.
This electromagnetic induction heating type fixing unit is an apparatus which is to subject an electromagnetic induction heating member to a magnetic field using a coil to thereby generate an eddy current, and which heat-fixes a not-yet-fixed toner image onto a recording material by using the electromagnetic induction heating member and applying heat onto the recording material as a material to be heated with Joule's heat based on the eddy current.
Inducing electromagnetic induction separately requires a high-frequency power supply circuit for passing an alternation current through the coil.
FIG. 7 is a block diagram schematically showing an arrangement of an induction heating apparatus using a high-frequency power supply circuit.
Generally, in a high-frequency power supply circuit 900, because it is necessary to switch a large amount of power at a high speed, there are employed resonant type inverter switching element, which makes it possible to reduce a switching power loss occurring when switching elements is turned on or turned off. This resonant inverter type switching element realizes zero-cross switching by the use of a resonance phenomenon between a resonant coil (induction heating coil) and a resonant capacitor.
In FIG. 7, reference numeral 9a designates a fixing roller; 900, a high-frequency power supply circuit; and 9c, an induction heating coil for inducing an induced current on the fixing roller 9a. Reference symbol TH1 designates a temperature sensor for detecting a temperature of the fixing roller 9a. 
The above-mentioned high-frequency power supply circuit 900 is constituted of a rectifying circuit 200 comprising diodes D1 to D4 for rectifying an alternating current power supplied from a power supply, a capacitor C1 connected between a noise filter NF1 which is connected to output terminals of the rectifying circuit 200, a capacitor C2 parallelly connected to the induction heating coil 9c, a electric power switching element TR1 (constituted by IGBT) connected to induction heating coil L1 in series, a diode D5 parallelly connected to the electric power switching element TR1, and a resonant control circuit IC1 for receiving a signal output from a temperature detecting circuit 7 based on a signal detected by the temperature sensor TH1, and outputting a control signal to the electric power switching element TR1. Further, the resonant control circuit IC1 is constituted by a one-shot pulse generating circuit 110, and a comparison circuit 130 for comparing an output signal of the one-shot pulse generating circuit 110 with an output signal of the temperature sensor TH1. Using such a high-frequency power supply circuit 900 enables a high-frequency alternating current power to be generated
A description will be given of an operation of the induction heating apparatus constructed as above.
When it receives a heating command signal, the high-frequency power supply circuit 900 generates a high-frequency alternating current power of about 20 to 100 kHz at its output terminal, thereby causing the coil 9c to receive an alternating current power to thereby generate an alternating magnetic field.
On this occasion, the alternating current power applied to the coil 9c is usually about 200 to 300 W, and possibly several KW at most, which are, however, varied depending on the size of the fixing roller 9a. An alternating magnetic field generated on the above-mentioned coil 9c due to the alternating current power applied thereto causes the fixing roller 9a to generate an eddy current to generate heat. This heat generation of the fixing roller 9a due to the electromagnetic induction operation causes the fixing roller 9a to increase in temperature.
Here, the temperature sensor TH1 for measuring the temperature of the fixing roller 9a monitors the temperature increasing of the heating roller as needed, and then the detected temperature of the fixing roller 9a is fed back to the resonance control circuit IC1. The above-mentioned induction heating power supply 1 compares the detected temperature with a predetermined target temperature, and hence makes the temperature of the fixing roller 9a constant in a proportional control manner or a commonly called PDI control manner of reducing the high-frequency electric power as the detected temperature comes close to the predetermined target temperature. Use of such a high-frequency power supply circuit 900 provides the induction heating.
However, the high-frequency power supply circuit 900 emits radiation noises because it passes a high-frequency alternating current through the coil. The radiation noises transmit from a print wiring pattern of a circuit and the like directly to a space and that out of such noises, their high-frequency components are easily transmitted; therefore, high-frequency power supply circuits as described above unfavorably transmit radiation noises more than usual circuits.
To this end, when an electromagnetic induction heating type fig apparatus is employed, surrounding the high-frequency power supply circuit using a metal box makes it possible to reduce the radiation noises.
But, even if a method of surrounding the high-frequency power supply circuit using the metal box is used, the radiation noises unfavorably leak from openings of the metal box through which cables electrically connect between the high-frequency power supply circuit and the coil, and possibly from their terminals disposed at the outside of the metal box.